The continuous or partially discontinuous preparation of MDA is disclosed, for example, in EP 1 616 890 A1, U.S. Pat. No. 5,286,760, EP-A-451442 and WO-A-99/40059. The acidic condensation of aromatic amines and formaldehyde to form diamines and polyamines of the diphenylmethane series proceeds in a plurality of reaction steps.
In the aminal process, formaldehyde is firstly condensed with aniline in the absence of an acid catalyst to form aminal, with water being eliminated. The rearrangement to form MDA is then carried out in the presence of an acid catalyst in a first step to form para- and ortho-aminobenzylaniline. The aminobenzylanilines rearrange in a second step to form MDA. Main products of the acid-catalyzed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and also higher homologs.
In the neutralization process, aniline and formaldehyde are converted directly in the presence of an acid catalyst into aminobenzylanilines which subsequently react further to form the two-ring MDA isomers and MDA homologs having more than two rings. Regardless of the process variant for preparing the acidic reaction mixture, the work-up thereof is commenced according to the prior art by neutralization by means of a base. The neutralization is usually carried out at temperatures of, for example, from 90° C. to 100° C. without addition of further substances. (H. J. Twitchett, Chem. Soc. Rev. 3(2), 223 (1974)). However, it can also be carried out at a different temperature level, for example in order to accelerate the degradation of interfering by-products. Hydroxides of the alkali and alkaline earth elements are suitable as bases. Preference is given to using aqueous NaOH.
After the neutralization, the organic phase is separated from the aqueous phase in a separation vessel. The organic phase containing crude MDA which remains after the aqueous phase has been separated off is subjected to further work-up steps, for example washing with water (base wash) in order to wash residual salts out of the crude MDA. The crude MDA which has been purified in this way is subsequently freed of excess aniline, water and other materials present in the mixture (e.g. further solvents) by suitable methods such as distillation, extraction or crystallization. The work-up customary in the prior art is disclosed, for example, in EP 1 652 835 A1, page 3, line 58 to page 4, line 13, or EP 2 103 595 A1, page 7, lines 21 to 37.
EP 2 486 975 A1 discloses the use of a specific mixer-reactor in the preparation of MDA. It is stated that a local excess of formaldehyde can lead to formation of network-like polymers. However, the patent application gives no details regarding the configuration of the start-up of the reaction, i.e. the commencement or resumption of the process after any interruption. In particular, the patent application does not teach that the “A/F ratio” (the molar ratio of aniline to formaldehyde) during the start-up procedure should be above the A/F ratio during normal operation.
EP 1 616 890 A1 teaches that aniline and formaldehyde are firstly reacted in the absence of the acid catalyst to form aminal and the aminal is subsequently admixed with the acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and at water contents of the acidic reaction mixture obtained in this way of from 0 to 20 percent by weight. In particular, the water is firstly at least partly removed from the aminal after the condensation of formaldehyde and aniline to form the aminal, with a water content of from 0 to 5 percent by weight in the aminal being set, and the aminal is subsequently admixed with an acid catalyst and is reacted further at temperatures of from 20° C. to 100° C. and at water contents of the acidic reaction mixture obtained in this way of from 0 to 20 percent by weight. Thus in this way, mixtures of diamines and polyamines of the diphenylmethane series having degrees of protonation of <15%, preferably from 4% to 14%, particularly preferably from 5% to 13%, can be prepared. Here, the degree of protonation is in the case of monoprotic acid catalysts (such as hydrochloric acid) the molar ratio of the amount of acid catalyst used and the molar amount of amine functions present in the reaction mixture.
The patent application gives no details regarding the procedure during the start-up procedure of an industrial production plant. The examples given are laboratory experiments. In particular, this patent application does not teach that the A/F ratio during the start-up procedure should be above the A/F ratio during normal operation.
EP 0 283 757 A1 is likewise concerned with the preparation of MDA. The process described is characterized by the addition of aniline-free MDAs to aminobenzylamines formed by condensation of aniline and formaldehyde before the heat-induced rearrangement reaction of the latter. Example 2 describes a mode of operation in which a small part of the MDA formed is recirculated to the rearrangement reaction (cf. also claims 6 and 8). In other words: the configuration of an MDA plant in continuous normal operation is described. Details regarding the procedure during start-up of an MDA plant are not described; in particular, there is no information regarding the A/F ratio during start-up compared to the A/F ratio during the reaction.
WO-A-99/40059 teaches that, to prepare methylenedi(phenylamine) by reaction of aniline with formaldehyde in the presence of acid catalysts in a semicontinuous process, aniline and optionally acid catalyst are initially charged, formaldehyde and optionally acid catalyst is fed through a mixing device into a circuit in which aniline, optionally acid catalyst and optionally previously added formaldehyde are circulated and, after introduction of at least 50% of the total amount of formaldehyde to be fed in, the reaction mixture is brought to a temperature of greater than 75° C. The addition up to an amount of at least 50% of the total amount of formaldehyde to be fed in is carried out at a temperature of the reaction mixture in the circuit of from 20° C. to 75° C.
None of the abovementioned documents of the prior art suggests using an A/F ratio which deviates from that during normal operation during start-up of the reaction for preparing MDA. Thus, it is quite customary in the prior art to use A/F ratios which are above that prescribed by the stoichiometry of the reaction (2:1) during normal operation. However, the prior art does not in any way suggest maintaining even greater A/F ratios during start-up.
The quality of a process for preparing MDA is defined firstly by the content of undesirable by-products of the reaction in the product. Secondly, the quality of a process is defined by the total process from start-up, normal production to running-down of the process to be able to be operated without technical production failure or problems which require intervention in the process and by losses of starting materials, intermediates or end product not occurring.
Such problems can, for example, occur during start-up of the aminal reaction. Such problems can be, for example, the formation of high molecular weight solids which lead to caking and blockages on the equipment (aminal vessel, cooler and separator and conduits).
Although it is possible to prepare MDA in high yield in the described processes of the prior art without the quality of the end products suffering, only processes which are in normal operation are described. Start-up processes up to attainment of a stable operating state at the desired load (known as “start-up time”) are not taken into account. A semicontinuously or continuously operated industrial process cannot, proceeding from a production plant which is not in operation (e.g. after a stoppage for maintenance purposes), be brought back to the process parameters before the production stoppage in an instant. Starting materials and apparatuses have to be heated up, apparatuses may have to be made inert, the supply of starting material to the apparatuses is gradually increased to the desired value. Start-up and running-down times occur frequently on an everyday basis in industrial processes and are not necessarily associated with opening or other mechanical intervention in a reactor or another apparatus of the plant, but can also be connected with the shutting down and restarting of the production plant for various other reasons, e.g. lack of raw material. These start-up times are characterized in practice by deviations in the desired molar ratio of aniline to formalin being able to occur.